Today, thin film photoelectric converters have become in wider use, and crystalline silicon photoelectric converters including crystalline silicon photoelectric conversion units have been developed in addition to amorphous silicon photoelectric converters including conventional amorphous silicon photoelectric conversion units. And moreover, hybrid type thin film photoelectric converters having these units stacked therein have also been put in practical use. A term “crystalline” as used herein represents also a state including polycrystalline state and microcrystalline state. Terms “crystalline” and “microcrystalline” as used herein represent a state partially including amorphous regions.
A thin film photoelectric converter generally comprises a transparent electrode film sequentially stacked on a transparent substrate, one or more semiconductor thin film photoelectric conversion units, and a back electrode film. And the one semiconductor thin film photoelectric conversion unit includes an i type layer sandwiched by a p type layer and an n type layer.
On one hand, an i type layer occupying a great portion of a thickness of the photoelectric conversion unit is made of a substantially intrinsic semiconductor layer, photoelectric conversion is mainly formed within this i type layer, and therefore the layer is referred to as a photoelectric conversion layer. For larger absorption of light and larger generation of photo current, this i type layer preferably has a greater thickness, but a greater thickness more than needed will result in increase in cost and time for the film-formation.
On the other hand, a p type layer and an n type layer are referred to as a conductive layer, and these layers serve to produce diffusion potential within the semiconductor thin film photoelectric conversion unit. A magnitude of this diffusion potential influences a value of an open-circuit voltage as one of properties of the thin film photoelectric converter. However, these conductive layers are inactive layers with no contribution to photoelectric conversion. That is, light absorbed by impurities doped in the conductive layers does not contribute to power generation, resulting in a loss of light. Consequently, the p-type and n-type conductive layers preferably have a thickness as small as possible in a range for providing a sufficient diffusion potential.
For this reason, in a semiconductor thin film photoelectric conversion unit or a thin film photoelectric converter, when the material of the i type layer occupying a major portion is made of amorphous silicon, the device will be referred to as an amorphous silicon thin film photoelectric converter or an amorphous silicon photoelectric conversion unit. And when the material of the i type layer is made of crystalline silicon, it will be referred to as a crystalline silicon photoelectric converter or a crystalline silicon photoelectric conversion unit. This expression is not dependent on whether materials of the conductive layer included are amorphous or crystalline.
Known methods of improving conversion efficiency of a thin film photoelectric converter involve stacking two or more photoelectric conversion units in tandem. In this method, a front unit including a photoelectric conversion layer having a wider energy band gap is disposed closer to a light incident side of the photoelectric converter, and behind it disposed is a rear unit including a photoelectric conversion layer (of a Si—Ge alloy, for example) having a narrower band gap. This configuration thereby enables photoelectric conversion over a wide wavelength range of incident light to improve conversion efficiency of the entire device.
In such tandem type thin film photoelectric converters, a device including stacked amorphous silicon photoelectric conversion units and crystalline silicon photoelectric conversion units are referred to as a hybrid type thin film photoelectric converter.
For example, wavelengths of light that may be converted into electricity by an i type amorphous silicon are up to approximately 800 nm in a longer wavelength side, but an i type crystalline silicon can convert light with longer wavelengths of approximately 1100 nm into electricity. Here, on one hand, in amorphous silicon photoelectric conversion layers comprising amorphous silicon having a larger light absorption coefficient, a thickness of not more than 0.3 micrometers is enough for sufficient light absorption for photoelectric conversion. On the other hand, however, in a crystalline silicon photoelectric conversion layer comprising crystalline silicon having comparatively smaller light absorption coefficient, it is preferable to have a thickness of not less than about 2 to 3 micrometers in order to fully absorb light with longer wavelengths. That is, a crystalline silicon photoelectric conversion layer usually needs approximately 10 times as large thickness as compared with that for an amorphous silicon photoelectric conversion layer.
In thin film photoelectric converters, needed are devices having larger dimension for larger electric generating capacity and improvement in productive efficiency. There are various problems in realizing a large sized device, for example, Japanese Patent Laid-Open No. 2002-319692 official report discloses a following technique. When a transparent substrate having a transparent conducting layer formed, using plasma CVD device, on one principal surface and having an dimension of not less than 1200 cm2 is held with a substrate holder, and is made to face with an electrode to form a crystalline silicon photoelectric conversion layer with a power flux density of not less than 100 mW/cm2, the substrate holder and the transparent conducting layer on a front face of the transparent substrate are electrically insulated to control abnormal discharge between the substrate holder and the transparent conducting layer on the front face of the transparent substrate. It is assumed that this abnormal discharge occurs, when an amount of electric charge accumulated in the transparent conducting layer exceeds a considerable quantity in escaping of the electric charge held in the transparent conducting layer to the substrate holder. Since a charge quantity escaping at once to the substrate holder is dependent on “dimension of substrate/circumference length of substrate”, this value is dependent on a substrate size. The official report describes that when a substrate size is large, specifically, when the substrate size is not less than 1200 cm2, a charge quantity escaping at once exceeds a certain steady value, and then the abnormal discharge easily breaks out.
Thin film photoelectric converters with a large dimension are usually formed as integrated thin film photoelectric converters. Generally an integrated thin film photoelectric converter is stacked on a transparent substrate, and the converter has a structure having a plurality of photoelectric conversion cells comprising a transparent electrode film, one or more semiconductor thin film photoelectric conversion units, and a back electrode film, each having a belt-shape connected in series.
Here, description of an integrated thin film photoelectric converter will be given referring to drawings. Identical referential numerals will be provided with an identical member in each Figure, and overlapping description will be omitted.
FIG. 1 is a schematic plan view showing an integrated thin film photoelectric converter 1. Still more detailed description about the integrated thin film photoelectric converter 1 shown in FIG. 1 will be given. FIG. 2 is a schematic sectional view showing the integrated thin film photoelectric converter 1. The integrated thin film photoelectric converter 1 shown in FIG. 2 is a hybrid type thin film photoelectric converter, and a photoelectric conversion cell 10 has a structure wherein a transparent electrode film 3, an amorphous silicon photoelectric conversion unit 4a provided with an amorphous silicon photoelectric conversion layer, a crystalline silicon photoelectric conversion unit 4b provided with a crystalline silicon photoelectric conversion layer, and a back electrode film 5, a sealing resin layer 6, and an organic protective layer 7 are sequentially stacked on a transparent substrate 2. That is, this integrated thin film photoelectric converter 1 perform photoelectric conversion of light entered from a transparent substrate 2 side by semiconductor thin film photoelectric conversion units 4a and 4b that form a hybrid type structure.
As shown in FIG. 2, a first, and a second isolation grooves 21 and 22 for dividing the thin film, and a connection groove 23 are provided in the integrated thin film photoelectric converter 1. These first and second isolation grooves 21 and 22, and the connection groove 23 are mutually parallel, and extend in a direction perpendicular to a page space. A boundary between adjacent photoelectric conversion cells 10 are specified by the second isolation groove 22.
The first isolation groove 21 divides the transparent electrode film 3 corresponding to each photoelectric conversion cell 10, and has an opening in an interface between the transparent electrode film 3 and the amorphous silicon photoelectric conversion unit 4a, and a surface of the transparent substrate 2 as a bottom. This first isolation groove 21 is filled with an amorphous material constituting the amorphous silicon photoelectric conversion unit 4a, and the material electrically insulates the adjacent transparent electrode films 3 from each other.
The second isolation groove 22 is provided in a position distant from the first isolation groove 21. The second isolation groove 22 divides the semiconductor thin film photoelectric conversion units 4a and 4b, and the back electrode film 5 corresponding to each photoelectric conversion cell 10, and the groove 22 has an opening in an interface between the back electrode film 5 and the sealing resin layer 6, and it has a surface of transparent electrode film 3 as a bottom. This second isolation groove 22 is filled with a sealing resin layer 6, and the resin electrically insulates the back electrode films 6 from each other between the adjacent photoelectric conversion cells 10.
The connection groove 23 is provided between the first isolation groove 21 and the second isolation groove 22. This connection groove 23 divides the semiconductor thin film photoelectric conversion units 4a and 4b, and has an opening in an interface between the crystalline silicon photoelectric conversion unit 4b and the back electrode film 5, and a surface of the transparent electrode film 3 as a bottom. This connection groove 23 is filled with a metallic material constituting the back electrode film 5, and the metallic material electrically connects one of the back electrode film 5 of the adjacent photoelectric conversion cells 10 with another transparent electrode film 3. That is, the connection groove 23 and the metallic material charged therein play a role in connecting in series the photoelectric conversion cells 10 aligned on the substrate 1 with each other.
By the way, in such an integrated thin film photoelectric converter 1, since the photoelectric conversion cells 10 are connected in series, a current value of whole of the integrated thin film photoelectric converter 1 during photoelectric conversion becomes equal to a current value of a photoelectric conversion cell 10, in a plurality of photoelectric conversion cells 10, having the minimum photo current generated in photoelectric conversion. And, as a result an excessive photo current in other photoelectric conversion cells 10 makes a loss. Then, investigations have been made for keeping uniform a quality of a film in a surface of the crystalline silicon photoelectric conversion unit 4b. That is, in the integrated thin film photoelectric converter 1 including the crystalline silicon photoelectric conversion unit 4b, in order to reduce the electric current loss mentioned above, efforts have been made to acquire high photoelectric conversion efficiency by reducing formation of an area generating only a small photo current due to difference of crystallinity in the crystalline silicon photoelectric conversion layer, and furthermore by making in-plane quality of the film uniform.
At this time, areas having a smaller photo current generated in the crystalline silicon photoelectric conversion layer may be distinguished from normal areas by visual observation of a side of the film surface after formation of the crystalline silicon photoelectric conversion unit 4b, and it may be observed as a whitish discoloring areas. This phenomenon is attributed to crystallinity difference in the crystalline silicon as a material of the crystalline silicon photoelectric conversion layer. On one hand, sufficient crystallization was not achieved in whitish discoloring areas, and the areas include not only crystalline silicon but much amount of amorphous silicon to give whitish and cloudy appearance, resulting in small amount of photo current. On the other hand, since they are fully crystallized, normal areas are observed as areas having gray appearance without whitish cloudiness, giving larger amount of photo current generated as compared with that from the whitish discoloring areas.
Japanese Patent Laid-Open No. 11-330520 official report discloses that in case of comparatively thin film-formation of an amorphous silicon photoelectric conversion layer, use of a higher pressure within a reaction chamber not less than 667 Pa (5 Torr) enables film-formation of a thicker crystalline silicon photoelectric conversion layer with high quality at a higher speed, instead of a conventionally used pressure of not more than 133 Pa (1 Torr) within a plasma reaction chamber, but the patent fails to provide description about such a whitish discoloring areas.
However, in a hybrid type thin film photoelectric converter or a crystalline thin film photoelectric converter having a dimension of not less than 600 cm2, there has been shown a problem that when the above-mentioned whitish discoloring areas do not exist in the crystalline silicon photoelectric conversion layer at all, areas giving a smaller photo current does not exist, and therefore a short circuit current increases with increase in light sensitivity, but an open-circuit voltage and fill factor will drop.